Since polypropylene resins are comparatively cheap and have various superior characteristics, they have been used in diversified fields. However, improvement in the impact-strength, particularly impact-strength at low temperatures has been required of the resins. Many processes for solving the above problem have been proposed so far. In general, a process for producing a propylene block copolymer wherein a propylene homopolymer component is first formed and then a component of an ethylene-propylene random copolymer is introduced thereto has been used. Whereas the propylene block copolymer has an improved impact-strength as compared with propylene homopolymers, it has a lowered rigidity, hardness and heat-resistance. As the mean for improving the demerits, a method has been proposed in Laid-open Japanese Patent Publication No. Hei 5-117342 wherein the ratio of the melt flow rate of the polymer obtained in the first stage to that obtained in the second stage are specified, and they have been broadly employed in various industrial fields such as automobiles and household appliances.
In recent years, thinning and lightening of injection-molded products or extrusion-molded products have been required from the viewpoints of saving resources and energy. However, polymers having a superior moldability while satisfying characteristics such as rigidity, hardness, heat-resistance, and impact-strength have not yet been provided. In order to obtain thinned and lightened injection-molded products or extrusion-molded products, it is necessary to increase the melt-flow rate of propylene block copolymers. However, when the melt-flow rate was increased, such a problem was raised that the rigidity, toughness and impact-strength were reduced, while the moldability was improved. Even in the case of the Laid-open Japanese Patent Publication No. Hei 5-117342 mentioned above, improvement in this respect is insufficient. For instance, when MFR (i) exceeded 50 g/10 minutes, Izod impact strength was extremely lowered. It is evident that in order to increase the melt flow rate of propylene block copolymers, it is sufficient to use propylene polymers having a low molecular weight. However, the subject of keeping the copolymers, at the same time, from causing the decrease in rigidity, toughness, and impact-strength has not yet been solved.
On the other hand, a polypropylene resin containing a propylene polymer having a position of main dissolution peak (T max) according to a temperature-rise fractionation method of 117.degree. C. or 118.degree. C. or higher, and a half-value width (.sigma.) of the peak of 3.4 degree or 4.0 degree or lower has been proposed in Laid-open Japanese Patent Publication Nos. Hei 5-5010, Hei 5-9218, Hei 5-9219, and Hei 5-32723 with a purpose of improving the rigidity and heat resistance of polypropylene, and also it has been described therein that a nucleator (nucleating agent) may be blended in the resin, if necessary. However, it was unable to regard the impact-strength, toughness, and rigidity of the resulting polypropylene resin as sufficient.
The subject of the present invention is to provide propylene-ethylene copolymer compositions which have a high rigidity, a high toughness, and a high impact strength, and are superior in moldability, and to provide a process for producing such compositions.